1. Field of the Invention
The present invention relates to a ceramic substrate-supported zeolite membrane and a method for forming the supported zeolite membrane and more particularly a monolithic ceramic substrate having supported thereon a thin dense, zeolite membrane exhibiting with few defects and a method for their production.
2. Discussion of the Related Art
Zeolites typically are hydrothermally formed alumina silicates of Group I and Group II elements. They can be represented by the following empirical formula: M2/nO.AL2O3.xSiO.2H2O, wherein xe2x80x9cxxe2x80x9d is generally equal to or greater than two and xe2x80x9cMxe2x80x9d is a cation having a valency of xe2x80x9cnxe2x80x9d. Synthetic zeolites generally have a greater uniformity and purity than do natural zeolites, as well as a greater degree of reproducibility. As such synthetic zeolites are more reliably produced for industrial applications.
Synthetic zeolite membranes, including those grown on, or supported by ceramic supports have many uses. For example, they exhibit catalytic properties, which give them considerable industrial importance. Also, the crystallographic structure of synthetic zeolites makes them particularly suitable for use as molecular sieves and cation exchangers for separation.
Fabrication of practical zeolite membranes, including supported zeolite membranes, has long been a goal of separation and catalyst science. As such, there is considerable prior art which describes the preparation of supported zeolite catalysts; specifically many different types of zeolites have been deposited on many different types of supports. Generally, zeolite membranes are formed by sequentially immersing a porous support in solutions of different reactants and then exposing the support, having the reactant solutions disposed in the pores of the support, to conditions sufficient to cause a zeolite to form, thereby forming a zeolite membrane. However, sequentially immersing a porous support into various reactant solutions causes the distribution of reactions in the pores of the support to be irregular resulting in zeolite membranes having significantly limited quality. For example, U.S. Pat. No. 4,800,187 (Lachman) discloses a method of crystallizing strong-bound zeolites on the surfaces of a monolithic ceramic substrate by hydrothermally treating the substrate in the presence of active silica, with a caustic bath to zeolitize the silica. As disclosed therein, the active silica can be present as a component of the caustic bath, in the form of a dried coating pre-deposited on the substrate, or as separate phase of the monolithic substrate, homogeneously dispersed within the ceramic material.
For a zeolite membrane to be practical, it must have high flux as well as selectivity. Obtaining such a membrane has been difficult in the past because of defects in the zeolite film. Typically, membranes are grown using low alkaline synthesis routes as known in the art and involve the formation of several zones across the membrane thickness, involving the formation of larger crystals grown on top of smaller crystals. In several zones the crystals are not grown into a dense mat free of intercrystalline voids and therefore, in order to obtain a permselective zeolite membrane, the aforementioned zeolite layers comprised of these zones must be grown to an excessive thickness ( greater than 50 microns) to seal off voids and defects. This creates a great mass transfer resistance causing reduced flux. Obtaining functional zeolite membranes from high alkaline syntheses routes is difficult because the heterogeneous crystals in the membrane require an enormous membrane thickness to seal pinholes and void structures which lowers the membrane selectivity. The presence of such pinholes and voids is the cause of optical scattering in as synthesized high alkaline membranes.
One recent innovation as disclosed in WO 96/01686 details a composition useful for separations of molecules and catalytic conversions which comprises a substrate, a zeolite or zeolite-like material in contact with the substrate and a selectivity enhancing coating in contact with the zeolite. This selectivity enhancing coating provides two functions, both of which lead the enhanced selectivity of the zeolite composition: (1) a stabilization effect involving alleviating or dispersing mechanical stresses or deformations within the zeolite layer arising from harsh environments; and, (2) reparation effect involving the sealing of defects or voids in the zeolite layer.
U.S. Pat. No. 5,567,664 (Barri et al.) discloses membranes comprising crystalline zeo-type materials carried by a porous support, with the crystalline zeo-type material being essentially continuous over the pores of the support. The zeo-type crystalline material extends into, is crystallized directly from and bonds directly to this porous support. Although this reference discloses continuous zeolite-like membranes having a continuous layer of zeolite directly connected to the surface of a support, one difficulty arises in the complex method of forming this zeolite membrane. The complex method comprises immersing at least one surface of a porous support in a synthesis gel capable of crystallizing to produce a crystalline zeo-type material. The method thereafter involves inducing crystallization of the gel so that the zeo-type material crystallizes on the support. After removing the support from the mix, the method involves repeating these steps one or more, preferably 3-10, times to obtain a zeo-type material membrane which is crystallized directly from and bonds directly to the porous support.
With the use of zeolite membranes becoming more diverse and sophisticated, and in spite of the fact that there has been significant research in the field of zeolite membranes and, it still remains a need, and would be highly desirable and an advancement in the art, to have a simpler, more economical method of forming supported zeolite membranes, which are thin, and essentially continuous exhibiting very few defects.
It is an thus an object of the present invention to eliminate the above-mentioned problems, and to provide a thin, essentially continuous supported synthetic zeolite membrane and a simpler, yet effective process for forming the supported thin, essentially continuous zeolite membrane.
Specifically, one aspect of the invention is directed toward a supported zeolite structure comprising substantially-sintered monolithic porous ceramic substrate having coated thereon a uniformly thin, essentially continuous zeolite crystal layer covering at least one surface and associated pores of the porous ceramic material. The zeolite crystal layer is comprised of a single layer of zeolite crystals and is free of any growth enhancing, selectivity enhancing, or reparation layer and exhibits an oriented structure whereby the crystals exhibit a substantially columnar cross-section. Preferably, the porous ceramic substrate support comprises, on an analyzed oxide basis, 10-95%, by weight Al2O3, and exhibits a crystal phase assemblage selected from the group consisting of cordierite, mullite, crystobalite, alumina, and/or mixtures thereof.
The invention is further directed to a method for forming a zeolite membrane on the surfaces of a monolithic ceramic substrate, generally comprising the steps of providing porous ceramic substrate and hydrothermally treating the substrate with an aqueous solution comprising sodium oxide or sodium hydroxide and having an alumina concentration of less than about 0.01%. The pH of the solution and the treatment time and temperature are such that these reaction conditions result in partial dissolution of the alumina in the substrate and essentially no nucleation in the solution thereby resulting in growth of the zeolite which is essentially continuous over the porous support. The porous ceramic comprises between about 10-95% alumina, by weight, is partly reactive, thereby capable of providing the alumina raw material for forming the zeolite membrane. However, alkali content is relatively low so as to limit excess dissolution of alumina, therefore limiting the alumina content of the treatment solution.